Physics and Technology Forefronts

MIT Team Devises Scheme for Wireless Non-Radiative Energy Transfer

“I love not having to plug things in anymore, but sometimes the side effects get to me.”

Many consumers long for the day when they can recharge laptops, cell phones and other ubiquitous electronic gadgets without having to lug around a separate bulky charger for each. That day might be closer than we think. A team of MIT physicists, led by Marin Soljacic, has been investigating the physics of electromagnetic fields, and has devised a demonstrable scheme for using wireless energy to power future gadgets.

Soljacic isn’t the first to pursue this concept. In the late 19th/early 20th century, Nikola Tesla conducted experiments in which he was able to light gas discharge lamps from over 25 miles away, without using wires. The recent Hollywood blockbuster film, The Prestige, depicts a fictional Tesla using a form of wireless energy transfer to light hundreds of electric light bulbs planted in an open field some 25 miles from his energy source.

That scene is based on contemporary accounts of such an incident. But Tesla had a more ambitious goal than merely powering light bulbs from a distance. He envisioned the construction of a global system of interconnected towers for wireless telegraphy, telephony, and power transmission, and began building a prototype, Wardenclyffe Tower in Long Island, New York. He was forced to abandon the project for lack of investment funds, and the structure was ultimately razed and sold for scrap metal.

However, the notion of the so-called “Tesla effect”–a type of high field gradient between electrode plates for wireless energy transfer –has endured. The effect uses high frequency alternating current, producing potential differences between two plates. Because of the surrounding magnetic flux, power can be transferred to a conducting receiving device–such as Tesla’s wireless bulbs.

Charging Ahead

Sometimes scientific inspiration can come from the most mundane unlikely sources. In Marin Soljacic’s case, it was his wife’s Nokia cell phone that inspired his approach to non-radiative wireless energy transfer. She continually forgot to recharge the device, and whenever the battery ran too low, the phone would emit a loud noise to alert the user to the impending battery death.

This often happened late at night or in the wee hours of the morning, to Soljacic’s annoyance. He thought it would be nice if the cell phone could recharge itself. To do so, however, would require a wireless means of transferring energy, with minimal energy loss. So he set about making that vision a reality. And the most obvious physical phenomenon for such a purpose, he decided, was strongly coupled resonance.

Soljacic grew up in Croatia before moving to the US after finishing high school, and is a fervent admirer of fellow Serbo-Croation, Nikoa Tesla. He is also a big fan of iRobot’s Roomba robotic vacuum cleaner, but laments, “It does a fantastic job, but after it cleans one or two rooms, the battery dies.” That’s why he owns several Roombas, but he envisions a day when Roomba could recharge using wireless energy transfer.

Physicists have long known that it is possible to transfer energy wirelessly using this powerful near-field effect. The oscillations of the magnetic field that surrounds a charged loop of metal can induce an electric current in another nearby metal loop, which can act as a battery or recharger. There are a few applications already for wireless recharging, most notably electric toothbrushes that use wireless transfer to recharge their batteries; the transcutaneous energy transfer (TET) systems used in some artificial hearts; and some cellular phones.

A British company called Splashpower has designed wireless recharging pads that also exploit electromagnetic induction. Users simply place their gadgets (cell phones, MP3 players) on the pads to charge them. BBC News quoted Splashpower co-founder James Hay pronouncing the MIT work interesting for future applications. “Consumers desire a simple universal solution that frees them from the hassles of plug-in chargers and adaptors,” he said, although challenges still remain to ensure efficient conversion of power into a form useful as input for electronic gadgets.

That is the primary stumbling block. Wireless energy transfer in such products is far from efficient: the emitted waves spread in all directions, and dissipate too rapidly over distance. Only a small fraction of the emitted energy is picked up by the receiver. That’s why most such approaches require the device to be extremely close to –or in direct contact with–the recharging pad or similar element.

So there was considerable buzz in the physics community when it was announced that Soljacic and his colleagues–Aristeidis Karalis and John Joannopoulos–had come up with a scheme for wireless non-radiative energy transfer. They investigated a special class of non-radiative objects that demonstrated long-lived resonances. When energy is applied to such objects, it remains bound to them as “tails” that flicker over the surface, rather than dissipating into space. The phenomenon is known as evanescent coupling, and strongly resembles quantum tunneling.

Specifically, Soljacic and his colleagues propose boosting the induced current via resonance, by introducing a short gap in a metal loop and attaching two small disks at either end. Such an object, when charged, has a natural resonant frequency–a byproduct of the current flowing back and forth along the loop from one disk to another. In theory, at least, two loops with the same frequency would mean that one should be able to receive energy from the other through the magnetic near field.

Soljacic’s key insight is that the close-range induction occurring inside a typical transformer could potentially transfer energy over short and mid-range distances, such as from one end of a room to another. A power transmitter would fill the space with a non-radiative electromagnetic field. This power would be picked up by a copper antenna that radiates at a frequency of 6.4 MHz. “Tails” of energy from the antenna would be able to “tunnel” up to 5 meters. This electricity would be detected by the gadget’s antenna, which must also resonate at 6.4 MHz, and that energy would be used to recharge the device.

Only objects designed to resonate with the frequency of that fieldwould be able to detect and absorb that energy. Any energy not transferred to the gadget would be reabsorbed by the source antenna. There would still be substantial losses, but the rate of transfer could reach tens of watts, sufficient to recharge a laptop within a few meters of the power source, according to Soljacic’s simulations. His team is now embarking on a series of experiments to test those simulations.

Currently, this method of wireless non-radiative energy transfer works over distances between three to five meters and shows between 30 to 60 percent energy efficiency –not ideal, but certainly an improvement over prior methods. Soljacic believes he can improve on these efficiencies, so that his approach can be adapted in the future for application in a factory, or scaled down to the microscopic or nanoscale realms. Thus, such a scheme could power not just small consumer electronics, industrial applications or electric vehicles (including helicopters), but also freely roaming nanorobots and macroscale robotic factory workers.

In fact, Soljacic foresees a day when there is a far-ranging infrastructure of such “midrange” energy nodes–akin to the wireless hot spots that provide laptop users with easy high-speed Internet access–in which entire buildings or other large areas would be able to automatically recharge wireless devices whenever they come within range. Perhaps one day it will be possible to send power to electric buses traveling along a highway.

While numbers in the team’s simulated calculations are encouraging, Soljacic cautions that it would be premature to start constructing homes without wall plugs of any kind. “We fairly strongly believe in our theory, based on previous experience. But experiments will be the ultimate judge,” he said.